1. Field of the Invention
The present invention relates to the monitoring of the operation of channels in cellular communications networks.
2. Description of the Related Art
The general layout of cellular communications networks is well known and will not be described in detail herein. Briefly, a network comprises a number of geographical areas or "cells" each provided with a base station (otherwise known as a "cell site") from which signals are sent to and received from transceivers (e.g. telephones) within the cell. The area of a cell is not strictly defined. The base station from which a particular telephone receives signals at any time will depend not only on the geographical location of the telephone but also other factors such as relative signal strengths between adjacent base stations.
A cellular network operator is allocated a number of frequency bands or channels on which it is permitted to send and receive signals. Each base station of the network operates on a selected group of the total number of permitted channels. Some base stations transmit and receive signals omnidirectionally. Others are subdivided into sectors which operate on a subgroup of the channels selected for the base station.
When a network is set up or a new base station is added to an existing network, channels must be allocated to cells or base stations in a manner which minimizes the probability of interference between simultaneous communications at the same frequency or adjacent frequencies. This is the task of a radio planner. For example, it might be assumed that a cell in Scotland could safely operate on the same channel as a cell in Cornwall since the distance between these locations would make interference unlikely. All kinds of geographical features contribute to the likelihood of interference including mountains and tall buildings. Rivers for example exhibit low attenuation of radio signals at cellular frequencies.
The radio plan is necessarily somewhat theoretical and is compiled mainly on the basis of "downlink" information, i.e. outgoing signals from the base station. Once a network or a new base station goes "live", two-way communications increase the probability of interference, mainly due to unforeseen to "uplink" problems. Such problems increase the probability of "dropped calls" on the network and reduce the overall performance of the network. The expression "dropped calls" is used to refer to telephone calls which are cutoff, from the user's point of view, when communication is lost and cannot be re-established within a reasonable period of time.
The occurrence of dropped calls and poor quality calls is probably the biggest disadvantage of mobile telephones versus "stationary" or "wired" telephones from the user's point of view and if one network operator could guarantee a lower dropped call rate and better quality calls than another, this would be attractive to mobile telephone users.
Interference which can ultimately result in dropped calls is mainly due to other telephones in operation although additional sources of interference include other electrical equipment, which sometimes goes "off tune" and interferes with mobile communications.
In order to distinguish between the active or wanted signal and an interference signal from another telephone in communication with another base station, all analogue cellular radio signals are superimposed with a SAT tone which acts as a signal identifier. A receiving base station will distinguish between the "wanted" signal and the interference signal by the SAT tone. The likelihood of two signals arriving at a base station on the same channel as well as having the same SAT tone is small.
Base stations are provided with equipment for monitoring incoming calls and interference and reporting problems on particular channels. If one particular channel is subject to an unduly high level of interference, it is "blocked out" and not reused until the level of interference has subsided. The base station equipment is able to provide a "block out" report which indicates the lack of availability of any particular channel due to excessive interference. If a particular channel is continuously blocked out it might be taken out of service altogether. The block out report would typically state the amount of time for which the channel was unavailable and the highest and lowest levels of interference during the block out period. The SAT tones associated with the interference might also be recorded. Since a channel cannot be blocked out during a call, the block out report does not identify dropped calls since these occur when a channel becomes susceptible to large interference after a call has been "connected". Thus, a block out report is an incomplete picture of the operation of any particular channel.
Network operators accept that a certain percentage of dropped calls are inevitable in any mobile communications network and manage the allocation of channels, at best, on the limited information available from a block our report, otherwise known as a "foreign carrier" report. The present invention is based on the realization that the performance of any channel in a communications network can be more accurately predicted on the basis of a statistical analysis of the signals occurring on the channel.